1. Field of the Invention
This invention relates to data storage systems employing magnetic recording media. More particularly, the invention is directed to tape drive systems and the monitoring of tape fly height during tape drive operations to facilitate data read/write accuracy.
2. Description of the Prior Art
By way of background, during operation in a tape drive data storage apparatus, a tape medium is transferred from (to) a supply reel to (from) a take-up reel while data is read from or written to the tape by one or more read/write heads. Typically (e.g., in a single reel cartridge design), the tape medium is spooled onto a reel and this reel is mounted inside a cartridge that is inserted into a slot in the tape drive so that the tape is in magnetic contact with the read/write heads. The tape from the cartridge winds onto a take-up reel that is located in the tape drive. The tape is advanced past the read/write head(s) by means of a pair of motors, one for each reel, which engage the reels when the cartridge is inserted and rotate the reels at a desired tape speed.
An important parameter to control during the service life of a tape drive is the fly height of the streaming tape medium relative to the read/write heads. As shown in FIG. 1A, fly height is the separation between the tape and the tape bearing surfaces of a head. Theoretically, the tape should be in intimate contact with the head such that the fly height is zero. However, the actual fly height can change during service as a result of contamination build up on the tape bearing surfaces and/or the tape, changes in head geometry due to wear, and other factors. An increase in fly height can produce a condition known as magnetic spacing loss in which the signal amplitude is reduced and uncorrectable data errors are produced. The increase in uncorrectable data errors over time is referred to as error rate drift.
A conventional technique is to measure changes in the tape magnetic spacing (also known as magnetic separation) relative to an initial reference condition. This is done using the well known Wallace Spacing Loss relationship to calculate the magnetic spacing change from measured amplitudes of readback signals derived from a series of tones that are prerecorded on the media. A disadvantage of using prerecorded tones to calculate magnetic spacing change is that the recorded tone information can decay over time such that calculation errors are introduced. In addition, read heads must be accurately aligned to the tracks containing the tones during readback. A further disadvantage of the conventional technique is that the magnetic spacing change calculation does not elucidate the tribological factors that go into creating the change. In particular, as shown in FIG. 1B, the change in magnetic spacing determined by the Wallace Spacing Loss relationship will not indicate change in true fly height if the read sensor (or the write coil element) becomes recessed from the tape bearing surfaces. In that case, the magnetic spacing (MS) is due to a combination of fly height (FH) and recession (R), as follows: MS=FH+R. Relatedly, the fly height is the difference between the magnetic spacing and the recession, as follows: FH=MS−R. A change in magnetic spacing is likewise due to changes in fly height and recession, as follows: ΔMS=ΔFH+ΔR. An additional consideration when using the Wallace Spacing Loss relationship is that a calculated change in magnetic spacing could be due to a change in the surface roughness and/or compliance of the tape medium, which produces a change in fly height alone.
Accordingly, it is desired to have an improved method of monitoring head-media interface conditions in a tape drive data storage system, including the ability to accurately monitor changes in fly height, head recession, and tape surface conditions. More generally, it would be advantageous to improve the methodology for monitoring head-media interface conditions in any data storage system that utilizes magnetic recording media, including not only tape drives, but disk drives as well.